Problem: In physics class, the girl to boy ratio is $9$ to $5$. If there are $36$ girls, how many boys are there?
A ratio of $9$ girls to $5$ boys means that there is a group of $9$ girls for every group of $5$ boys. If there are $36$ girls, then there are $4$ groups of $9$ girls each. Because we know the ratio is $9:5$ , there must be $4$ groups of $5$ boys each. There is a total of $20$ boys in physics class.